Tomography is the study of the internal properties
of a body by observing the behavior of rays passing through the body. Seismic
tomography uses mathematical modeling of P and S wave travel times to map
velocity perturbations in the interior of the Earth. The primary energy
source used in global seismic tomography is seismic waves generated by
earthquakes which pass through the Earth in all directions, and are recorded
on seismograms around the world. Inversion of arrival time data is used
to determine the speed of the waves at any given point in the Earth. Using
seismic tomography to interpret the internal structure of the Earth is
similar in technique to a CAT-scan. Computer assisted tomography (CAT)
uses X-rays transmitted through the body in many different directions.
A mathematical method is then applied to explain the loss in intensity
of the X-rays due to the varying absorptive properties of different parts
of the body. The comparison between CAT-scans and seismic tomography differs
because X-rays travel in straight paths, whereas the ray paths of sound
waves bend with changes in the velocity structure of the medium.

What are the uses of seismic tomography ?

Seismic tomography has several applications in exploration
and global geophysics. Crosshole transmission tomography is used to image
subsurface features between boreholes with greater accuracy than conventional
surface reflection methods. Seismic tomography can also be used to characterize
fractured bedrock, map groundwater reservoirs, and locate ore bodies. Global
seismic tomography is used to interpret the presence of ancient subducted
slabs, locate the source of hotspots, and model convection patterns in
the mantle.

What are the limitations of seismic tomography
?

Global seismic tomography is limited by the irregularity
in time and space of the source, and by the incomplete coverage of recording
stations. The primary source is earthquakes, which are impossible to predict
and only occur at certain locations around the world. In addition, the
global coverage of recording stations is limited due to economic and political
reasons. Because of these limitations, seismologists must work with data
that contains crucial gaps. Experimental data can not accurately replicate
conditions deep in the Earth's interior, therefore making comparisons with
real world data difficult. Another limitation in imaging deep structures
is attenuation and absorption of energy due to the long distances waves
travel through the Earth, which reduces the resolution which can be attained.
Due to the problem of attenuation, the minimum sizes of features in the
mantle which can be resolved are blocks 100-200 km on each side.

Mapping of velocity perturbations in the Earth's mantle
results in an indication of mantle temperature variation. Waves tend to
travel faster in colder regions than in hotter regions. This is due to
density contrasts related to temperature. Colder materials are more dense
than hotter materials, allowing them to transmit waves at a higher speed.
Figures 1 & 2 show maps of the mantle generated from tomographic data.
These map are color coded with red correlating to slower velocities and
blue indicating faster velocities. Looking at these maps, one can discern
a strong correlation between tectonic features and the velocity of waves.
Areas of younger, hotter material, such as actively spreading ridges, correspond
to red (slow) areas on the velocity map. Areas of old, colder material,
such as the interiors of continents, correspond to blue (fast) areas on
the velocity map. See Figures
1 & 2.

Velocity anisotropy and mantle convection.

Velocity anisotropy means that waves propagate
at different rates in different directions. This property can be used to
infer mantle convection patterns. The primary component of the mantle is
believed to be olivine. The anisotropic structure of olivine crystals allows
waves to propagate faster along the long (a) axis of the crystal than along
the short axes (perpendicular to a). Flow in the mantle due to convection
is believed to align the olivine crystals with their long axis oriented
in the direction of the flow. Therefore, mantle convection patterns can
be mapped using velocity perturbations, because the waves will travel faster
along paths where crystal lattices have been aligned due to flowing of
mantle material. Verically and horizontally polarized shear-wave velocities
determined by tomography indicate that vertical flow is dominant under
ridges and subduction zones, and horizontal flow dominates under cratonic
areas.

As seen in figures 1 & 2, seismic tomography can be
used to delineate plate boundaries. Tomographic velocity maps of the mantle
show areas of high and low velocity, and additionally with temperature
variation and anisotropy, these velocity variations have been used to infer
the depth of mountain roots and ancient subducted slabs. Three dimensional
models have been generated to show the velocity structure of the mantle
underneath the continents. Near the center of the continents, and under
mountain ranges, velocity contours can be seen to curve. This seems to
support the theory of isostasy, or compensation of a thick unit
of continental crust by a large mass of buoyant material beneath. Tomography
has also been used to infer the location of ancient subducted slabs. See
figure 3. Figure 4 shows P and S wave models which illustrate
global high and low velocity areas. High velocity areas, where ancient
subduction zones are believed to have existed, have been interpreted to
represent remnants of old slabs of dense subducted oceanic crust. See
figure 4. Another application of seismic tomography to plate
tectonic theory lies in the imaging of low velocity zones below proposed
hot spots,or areas of constant volcanic activity. Hot spots
are believed to originate deep in the mantle. Images generated from tomographic
studies have shown low velocity zones extending deep into the mantle under
the Hawaiian and other hot spots. This could indicate higher temperatures
in these areas and possibly melt generation at depth. This can also help
to disprove some proposed hot spots which do not correlate to low velocity
zones.

Seismic tomography has potential for many aspects of exploration
geophysics. These include shallow, high resolution techniques used in environmental
and economic exploration, such as crosshole transmission tomography used
to image the flanks of salt domes, and better evaluate ore bodies between
boreholes. The applications used to image the deep structure of the Earth
are limited in their usefulness, and can only be realistically improved
by increasing the density of seismogram coverage. The only other option
to improve the available data is to detonate large explosions, such as
nuclear bombs, at locations where gaps in the data exist.